Synergistic antioxidant composition



atented Nov. 3, isg

SYNERGISTIC AN TIOXIDAN T COMPOSITION Eugene FQHill and Margaret L.Welp, Detroit, Mich., assignors to Ethyl Corporation, New York, N. Y.,a, corporation of Delaware No Drawing. Application December 24, 1949,Serial No. 135,044

13 Claims.

This invention relates to the stabilization of organic materialsnormally susceptible to deterioration. More particularly our inventionrelates to the inhibition of attack by oxygen and the prolongation ofthe useful life of oxygen-sensitive materials. This invention is furtherdisclosed in applicants co-pending application, Serial Number 277,336.

Hydrocarbon fuels for internal combustion engines may be broadlyclassified into three categories, according to the use for which theyare intended; fuels for automotive spark ignition engines, fuels foraircraft spark ignition engines, and fuels for compression ignitionengines.- Although each such fuel is composed essentially ofhydrocarbons, the stability characteristics during the manufacturingprocess and subsequent storage and use, particularly in the presence ofoxygen, differs considerably for each type. For example, typicalautomotive fuels contain straight and branched chain aliphatics,

olefins, naphthenes and some aromatics, while typical aircraft fuelscontain smaller proportion of olefins. In recent years fuels forcompression ignition engines have contained an increased proportion ofcracked stocks, resulting in a higher olefin content and consequentincrease in the susceptibility to gum formation. The effect of thedeterioration of the fuel upon each type of engine may differ, butequivalent processes of deterioration occur in each fuel. For example,the formation of gum in fuels designed for use in spark ignition enginesinterferes with normal operation of the ignition system and valves,While the formation of such gummy materials in compression ignitionengine fuels interferes with the normal operation of the fuel filtersand injectors in such engines.

In general, the hydrocarbons present in automotive gasolines are moresusceptible to degradation than those comprising aircraft fuels.However, both automotive and aircraft fuels are commonly blended withtetraethyllead before use. Such blending imposes a further point ofinstability in the finished fuel, since the tetraethyllead issusceptible to some deterioration by contact with oxygen during theblending, storage and handling operations, with consequent formation ofhaze, loss of some antiknock value, and lessened performance in theengine. This point of attack is often overlooked and is ordinarilyunimportant in automotive fuels, as the protective measures necessaryfor th base stock are usually more than sufficient to protect thetetraethyllead. If, however, a stabilizing ingredient were added whichis capable of protecting only the fuel, the attack upon thetetraethyllead would then become apparent. In aircraft fuels theprotection must center upon the antilmock additive, as the fuel itselfis relatively stable. Furthermore, this phase of the problem becomesrelatively more important in aircraft fuels, since the tetraethylleadcontent of such fuels is generally several times that present inautomotive fuels.

Heretofore, the protection of fuels for internal combustion engineseffectively against the two above-described separate but relateddeleterious efiects of contact with oxygen during the refining,manufacturing, blending, storage and handling operations has beenaccomplished only with difficulty. Furthermore, because of thespecifications imposed on such fuels by the rigid requirements orpresent day engines, particularly aircraft engines, it is essential thatany material used to protect such fuels against deterioration beeffective in extremely small quantities, on the order of one pound ofadditive per five thousand gallons of fuel, so that secondary problemsdo not arise through their use.

Similarly, synthetic and natural elastomers are susceptible toabsorption of oxygen with consequent destruction of certain usefulphysical properties and with the introduction of certain propertieswhich render articles manufactured from such elastomers of limitedutility. By absorption of oxygen such elastomers deteriorateprematurely, lose tensile strength and flexibility, and becomediscolored and embrittled. While certain materials have been proposedfor the protection of such elastomers from the deleterious action ofoxygen, most of such protective substances, as for example 18-naphthol,possess the serious disadvantage, particularly with respect to lightcolored stocks, that their own degradation products are themselvescolored and hence interfere with the color fastness of the stocks beingprotected.

Further examples of materials which must be protected from thedeleterious effects of oxygen include mineral oils, such as lubricatingoils, soaps, certain perishable foodstuffs, such as animal and vegetableoils and fats, and synthetic unsaturated organic materials. In general,such organic substances may require protection at any time during theprocesses of manufacturing, handling, storage and use when they becomeexposed to and absorb oxygen with deleterious effects.

It is therefore an object of our invention to provide means forprotecting such organic substances which deteriorate in or are affectedadversely by oxygen. It is a further object of our invention to providea class of mixture which provides the required protection against theformation of gummy oxidation and polymerization products of unstablehydrocarbons on contact with oxygen at reduced levels of additive.Another object of our invention is to provide means for increasedstabilization of hydrocarbon fuels for internal combustion spark, andcompression ignition engines during the manufacturing, han- I is also anobject of our invention to provide fuels containing tetraethyllead inwhich there is essen- I tially no loss in performance characteristicsdue to such deterioration of the tetraethyllead during blending, storageand handling. Likewise it is an object of our'invention to provide meansfor preventing embrittlemen't, discoloration, loss of tensile strengthand other harmful effects in elastomers during the milling, compounding,fabrication, storage, handling and use of such elastomer stocks. Afurther object of our invention is to provide means for protecting otherperishable natural or synthetic organic materials from the adverseeffects of contact with oxygen. Still further objects of our inventionwill appear from the description of our invention as hereinafterdisclosed.

The above objects canybe accomplished by practicing our invention whichcomprises adding to oxygen-sensitive organic materials a smallproportion of a composition comprising arylamine antioxidant materialsand substances derived from the class 'ofmercaptans.

We have made the discovery that certain organic mercaptans, themselveincapable of protecting organic materials fromdeterioration in thepresence of oxygen, have the property of greatly increasing theeffectiveness of previously known antioxidants of the arylamine type.Such mercaptans we refer to hereinafter as synergists. The mercaptans ofour invention include those which contain hydrocarbon radicals, selectedheterocyclic radicals, and certain functionally substituted hydrocarbonradicals. Among the hydrocarbon radicals which we have found to beeffective substituents of the mercaptans of our invention are alkyl,aryl and aralkyl. As examples of heterocyclic radicals effective in ourmercaptans we include the furf-uryl group and the imidazolidine group.The functionally substituted hydrocarbon radicals =01 our synergisticmercaptans include "carbalkoxy -groups, as in the alkyl thioglycolates.

We have found that the replacement of a portion of thearylamineantioxidant of a stabilized composition with :an equal weightof our synergistic mercaptans, themselves'incapable-of providing oxygenstabilization, results in a'stabilized composition which --is moreresistant to attack by oxygen. Furthermore, to attain a specified'degreeof oxygen stability--the required amount of our arylamine-mercaptanmixture is less than the required amount of the arylami-ne antioxidantalone. For convenience in referring to known antioxidants hereinafterthe following letters are assigned thereto:

The absorption of oxygen by hydrocarbon fuels can be measured directlyby the standard method of theAmerican Society of Testing Malterials fordetermination of the oxidation stability of gasoline ASTMdesignation:B52546, as fully described in Part III-A, ASTM Standards for 1946.According to this method the induction period is the period during whichthere is no absorption of oxygen by the test material as indicated by adrop in pressure, when placed in a testing bomb maintained at atemperature of 100 C. with an initial pressure of1100 pounds per squareinch gauge of oxygen. The induction period increase (IPI) is theincrease in the duration of this period caused by the addition of aprotective substance, and is a direct measure of the protection affordedby such additive. Thus, the longer the .IPI the'more effective is thestabilizer. On the contrary, certain substances exert a prooxidanteffect in which a negative-1P1 is obtained, that is, the duration of theinduction period, or period of no absorption of oxygen, is less than inthe absence of the additive.

It is well known in the art of protecting gasoline from oxidation thatthe susceptibility to oxidation of gasoline varies significantly withdifferent types of 'gasolines. Furthermore, it is likewise well knownthat the efiiciency of any antioxidant, and, therefore, the minimumconcentration required, will vary greatly from gasoline to gasoline.Therefore, in order to "show the general applicability of the compoundsof our invention to the solution of this problem, and at the same timenot present in detail thelarge amount of data so obtained, we havelisted in Tables I and II representative induction period increasesobtained with a variety of test gasolines. The gasolines used inobtaining the-data presented herein were all commercial blending stocksor finished gasolines and included the following types: Anaverage-response gasoline containing 20 per cent olefins and 14 per centaromatics, the remainder being paraflins and naphthenes; a gasolinecontaining approximately 38 per cent olefins, '33 per cent aromatics,and the remainder parafiin's and naphthenes; a gasoline containing 18per cent olefins and 24 per cent aromatics; a high-sulfur gasolinecontaining 0.21 per cent sulfur; and a gasoline containing 28 per centolefinsand '18 per cent aromatics with medium sulfur content.

The ASTM method employed to illustrate the activity of the compounds ofour invention as in Tables I and II is a reliable indication of theefficiency of a stabilizing material within the test limits of plus .10minutes andminus 10 minutes. Materials exhibiting an effect near theselimits are essentially inert. To obtain the results shown herein, 6milligrams of the additive was dissolved in milliliters of the gasoline.Where the solubility characteristics oftthe material were such that thisconcentration could not be obtained, a small amount of a 'solubilizingagent, such as ethyl or isopropyl alcohol, was added'in amount up toZper cent of the gasoline.

TABLE .I

Efiect on induction period increase ofgasolin'es Synergist IPI Amylmercaptan l 5 Dodecyl mercaptan -65 Furfuryl mercaptan -202-mcrcapto-imidazolidine +20 Isopropyl thioglycolate (induction periodmethodh.

From Table I it is clear that the synergizing components of theantioxidant mixtures of our invention are essentially inert or act aspro-oxidents. Other mercaptans of our invention which do not exhibit anantioxidant effect when employed alone in gasoline include allylmercaptan, t-butyl mercaptan, benzyl mercaptan,2-mercapto-4-methyl-imidazolidine, ethyl thioglycolate and butylthioglycolate.

The effectiveness of the mercaptans of our invention as synergists isshown in Table II, wherein the mercaptans were added to the testgasoline in admixture with known antioxidants. We have listed in TableII the increase in the induction period of the so-treated gasoline at atotal concentration of 6 milligrams of the synergistic mixture per 100milliliters of gasoline, and the increase in the induction period of thetest gasoline treated with the arylamine antioxidant alone at the sameconcentration of 6 milligrams per 100 milliliters of gasoline.

table. Further examples of the compounds of our invention which producea synergistic effect with antioxidant materials include allyl mercaptan,benzyl mercaptan, 3-propyl-2-mercaptoimidazolidine and octylthioglycolate.

We have demonstrated the efficiency of the synergistic mixtures of ourinvention in preventing undue formation of gum in automotive gasolinesby storing such gasolines containing them for long periods, anddetermining from time to time the gum content of the fuel. The quantityof gum so-formed was compared with that. formed in the presence of knownantioxidants and in the untreated gasoline. Two commercial motorgasolines, consisting of 27 and 38 per cent olefins and initiallycontaining 0.6 and 0.8 milligram of gum per 100 milliliters, wereemployed. For each demonstration duplicate amber quart bottles werefilled with one pint of the gasoline with or without an additive. Thebottles were stoppered and stored in the dark at a temper- TABLE IIEfiect on induction period increase of gasoline s t AntisPercenttolfIPI/ th gs E y No. ynergis oxiynergis n m". In 1 dant mixture 0!antioxidant gifigi g 1 Amyl mercaptan A 33 250 365 t-B utyl mercaptan A300 485 Dodecyl mercaptau A 25 300 470 Dodccyl mercaptan. A 50 235 325Furfuryl mercaptan. A 33 250 370 2-m crcapto-imidazoli A 33 250 380 7Ethyl thioglycolate A 25 300 480 8. Isopropyl thioglycolate B 33 340 4059. Isopropyl thioglycolate C 33 335 410 10.-.- Butyl thioglycolate A 25365 550 By reference to Table II the increase in protection afforded toan unstable gasoline at the same total weight of additive, achieved byreplacing a portion of the known antioxidant material by the mercaptansof our invention, is

apparent. Thus, for example, replacing 25 per cent of antioxidant A withan equal weight of t-butyl mercaptan increases the induction periodincrease of the fuel over 60 per cent. Stated ature of 110 F. Every fourweeks the bottles and their contents were cooled to room temperature andthe stoppers were removed for two hours to permit access to the air.Every 8 weeks a sample of the fuel mixture was removed and the dissolvedgum therein was determined by the socalled air-jet evaporation method,ASTM designation: D381-46, fully described in ASTM Standards for 1946,Part III-A.

TABLE III Eficct on gum in motor gasoline Increase in gunlingontent,mg./100 No. Additive 338- 5 8 16 24 32 eeks weeks weeks weeks (Gasoline#1) 1 Antioxidant A (%)+2- 3 1. 2 1. O 3. 8 3. 7

nzercaspto imidazolidine 2 Antioxidant A l. 5 2. 5 2. 3 5. 3 7. 3 3Noneun 3.1 15.0 109. 4 407. 9

(Gasoline #2) 4 Antioxidant A (50%)+ ethyl 3. 0 2. 3 3. 5 4. 5. 2

thioglycolate (50%). 5 Antioxidant A 1.5 1. 0 2. 5 6. 9 7. 3 6 None 8. 143. l 131. 6 326. 1

difierently, 6 milligrams of synergistic mixture No. 2 in Table II isequivalent to almost 10 milligrams of antioxidant A, although t-butylmercaptan alone is not an antioxidant. Thus, to obtain an IPI of 300, asin No. 2, only 3.8 milligrams of our mixture would be required,containing 2.85 milligrams of antioxidant A and 0.95 milligrams oft-butyl mercaptan. Similar considerations apply in each of the examplesof the As shown in Table III by incorporating the mercaptans of ourinvention, which by themselves are not antioxidants, into gasolinecontaining materials known to be antioxidants, a substantial reductionin the amount of gum formed during storage was obtained compared to thegasoline containing the antioxidant alone. In this comparison, oursynergists were used in addition to known antioxidants, and resulted inimproved storage stability of the fuel. As a further embodiment we havereplaced part of the antioxidant material with our synergists, therebyobtaining equivalent protection but utilizing for the purpose a smallerquantity of the antioxidant.

To illustrate the protection afforded to hydrocarbon solutions oftetraethyllead by the compounds of our invention we conducted a seriesof tests in which hot-acid isooctane containing 4.6 milliliters oftetraethyllead per gallon was heated at a temperature of 100 C. in astainless steel bomb with oxygen added to an initial pressure of 100pounds per square inch gauge. Under these conditions the pressure in abomb containing enly isooctane and a tetraethyllead antiknock mixtureunderwent a sharp drop after-four hours, indicating absorption of oxygenby the fuel mixture. Various amounts of known antioxidants and oursynergistic mixtures were thereupon added until it was determined atwhat minimum concentration the pressure in the bomb did not drop duringa period of 16 hours at a temperature of 100 C. Thus, the eifectiveconcentration shown in Table IV is the minimum quantity of additiverequired, expressed as milligrams per 100 milliliters of fuel, to affordat least a four-fold increase in the stability of the fuel.

By reference to Table IV it is clear that by replacing one-third of thearylamine antioxidant by a synergist of our invention, the mixture iseffective at one-half the concentration required for the antioxidantalone. tained with a synergist which, when employed alone, fails toprotect the fuel at over eleven times the efiective concentration of themixture of our invention.

A further class of organic substances sensitive to oxygen comprises theelastomers, natural and synthetic. To illustrate the utility of thesynergistic mixtures of our invention in protecting such substances weselected a natural rubber compounded into a typical tire-tread formula.One requisite of such stocks is that the desirable propertiesincorporated therein by careful selection of the compounding ingredientsand cure time shall be maintained during extended periods of storage oruse in the presence of oxygen. Comparison of various rubber stocks isbest carried out on stocks initially having the same state of cure. Themost reliable means for determining the state of cure is by the T-50test, ASTM designation: DEBS-4.0T, described in-the ASTM standards for1946, Part III-B. This test measures the temperature at which a testpiece recovers its elasticity when it is stretched at room temperature,frozen at a suificiently low temperature to cause it to lose its elasticproperties, and then gradually warmed. In practice the temperature notedis that at which the sample recovers to 50 per cent of the originalelongation and is, therefore, referred to as the T-50 value.

This result is obd testing and comparison were cured for a timesufiicient to have a T-50 value of +1 0., so that a valid comparison ofthe properties could be made. The accelerated aging was conducted by theprocedure of ASTM designation: 13572-42, described in the ASTM Standardsfor 1946, Part III-B, for a period of 96 hours at a temperature of 70C., with an initial oxygen pressure in the test bomb of 300 pounds persquare inch gauge on specimens showing the following composition:

Parts by weight Smoked sheet 100.00 Carbon black 45.00 Zinc oxide 5.00Stearic acid 3.00 Pine tar oil 2.00 Sulfur 3.00 Mercapto-benzothiazole0.65 stabilizing ingredient 1.00

To demonstrate the protection afiorded to the rubber by the mixtures ofour invention, the tensile strength and the ultimate elongation ofstocks prepared with the addition of a synergistic mixture of ourinvention were determined before and after aging. These properties werecompared with the same properties determined on an identical rubberstock protected by the arylamine antioxidant alone, and finally with astock not protected by an inhibitor. Both of these properties weredetermined by means of the test procedure of ASTM designation: 13412-41,fully described in ASTM Standards for 1946, Part HI-B. The tensilestrength is the tension load per unit cross-sectional area required tobreak a test specimen, while the ultimate elongation is the elongationat the moment of rupture of a test specimen. A decrease in the valuesfor either of these properties upon aging represents a decrease in theusefulness of the article fabricated therefrom, so that the degree towhich these properties are retained is a direct measure of the utilityof the protective substance.

By referring to Table V it is apparent that both Nos. 1 and 2 areeffective in promoting retention of the tensile strength and ultimateelongation over the control which contained no protective additive (No..3). However, by replacement of one-third of the known antioxidant bythe synergistic material of our invention, not only was protectionmaintained, but an increase in retention of the original physicalproperties resulted.

. The quantities of the mixtures of our invention incorporated in thematerials to be stabilized are not critical and depend largely upon thetype of material being stabilized and the conditions under which theexposure to oxygen occurs. For example, with gasolines, tetraethyllead,mineral In the examples that follow stocks for oils and similarmaterials the mixtures of our invention are preferably employed inconcentrations between the limits of approximately 0.1 and milligramsper 100 milliliters of material to be stabilized. For other materials,such as for example elastomers, both natural and synthetic, somewhatlarger amounts of the stabilizers of our invention are preferred and canbe tolerated. Thus, in such materials we employ between approximately0.1 and 2 parts of synergistic mixture per 100 parts of oxidizablematerial. Thus, our mixtures can be satisfactorily employed in a widerange of concentrations, and we do not intend that our invention berestricted to the specific quantities mentioned herein.

Furthermore, we do not mean to be restricted by the ratios ofsynergistic mercaptan to arylamine antioxidant employed in the specificembodiments of our invention disclosed herein by way of examples. Suchratios will be determined in part by the nature of the material to bestabilized, in part by the specific arylamine employed and in part bythe specific synergistic mercaptan. In general, however, we prefer toemploy between about 30 and 100 parts by weight of mercaptan to 100parts by weight of arylamine.

We have disclosed a number of preferred embodiments of our invention andillustrated several means whereby protection can be afforded to organicmaterials sensitive to attack by oxy- U gen. Our invention is notintended to be limited to the specific embodiments of our inventionherein or to the means described herein for obtaining the advantagespossible in employing our synergistic mixtures, as other methods ofpracticing our invention will be apparent to those skilled in the art.

We claim:

1. A synergistic antioxidant composition effective in inhibitingoxidation in oxygen-sensitive organic materials, said compositionconsisting essentially of a lower alkyl-p-phenylene diamine and a sulfurcompound selected from the group consisting of alkyl mercaptanscontaining from 4 to 12 carbon atoms, furfuryl mercaptan, the loweralkyl esters of thioglycolic acid, 2-mercapto imidazolidine and loweralkyl Z-mercapto imidazolidines, said sulfur compound being present inamount by weight between about per cent and about 50 per cent of saidsynergistic antioxidant compound.

2. The composition of claim 1 in which the arylamine antioxidant isN,N-di-sec.-buty1-p phenylenediamine.

3. The composition of claim 2 in which the sulfur compound is an alkylmercaptan containing from 4 to 12 carbons.

4. A synergistic antioxidant composition effective in inhibitingoxidation in oxygen-sensitive organic materials consisting essentiallyof 100 parts of N,N'-di-sec.-butyl-p-phenylenediamine and between aboutand 100 parts of t-butyl mercaptan.

5. The composition of claim 2 in which the sulfur compound is furfurylmercaptan.

6. The composition of claim 2 in which the sulfur compound is a loweralkyl ester of thioglycolic acid.

7. A synergistic antioxidant composition effective in inhibitingoxidation in oxygen-sensitive organic materials consisting essentiallyof parts of N,N-di-sec.-butyl-p-phenylenediamine and between about 30and 100 parts of ethyl thioglycolate.

8. The composition of claim 2 in which the sulfur compound is a loweralkyl 2-mercapto imidazolidine.

9. A synergistic antioxidant composition effective in inhibitingoxidation in oxygen-sensitive organic materials consisting essentiallyof 100 parts of N,N-disec.-butyl-p-phenylenediamine and between about 30and 100 parts of 3-propyl-2-mercapto imidazolidine.

10. A synergistic antioxidant composition effective in inhibitingoxidation in oxygen-sensitive organic materials consisting essentially.of 100 parts of N,N'-di-sec.-butyl-p-phenylenediamine, and between about30 and 100 parts of 2-mercaptoimid'azo1idine.

1'1. A composition consisting essentially of at least one hydrocarbonthat normally tends to deteriorate in the presence of oxygen, and aneffective amount of a synergistic antioxidant mixture of an arylamineantioxidant and a sulfur compound, as defined by claim 1, said sulfurcompound being present in amount between about 30 and 100 parts per 100parts of said ary1 amine antioxidant by weight.

12. A petroleum hydrocarbon fuel for internal combustion engines,normally subject to deterioration in the presence of oxygen, containingas a principal antioxidant ingredient a synergistic antioxidant mixtureconsisting essentially of an arylamine antioxidant and a sulfur compoundas defined in claim 1, said sulfur compound being present in amountbetween about 30 and 100 parts per 100 parts of said arylamineantioxidant, and said synergistic mixture present in amount betweenabout 0.1 to 15 milligrams per 100 milliliters of said hydrocarbon fuel.

13. A fuel for internal combustion engines containing petroleumhydrocarbons stable to oxygen, an antiknock additive substancecomprising tetraethyllead, the presence of which tends to increase thedeterioration of the resulting fuel blend in the presence of oxygen, andas a principal antioxidant ingredient the synergistic mixture of claim1, in amount between about 0.1 to '15 milligrams per 100 milliliters ofsaid fuel composition.

EUGENE F. HILL. MARGARET L. WELP.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,966,050 Sloane July 10, 1934 2,033,877 Burk Mar. 10, 19362,120,244 Dryer June 14, 1938 2,395,382 Walters Feb. 19, 1946 2,414,145Evans Jan. 14, 1947 2,443,569 Ruggles June '15, 1948

1. A SYNERGISTIC ANTIOXIDANT COMPOSITION EFFECTIVE IN INHIBITINGOXIDATION IN OXYGEN-SENSITIVE ORGANIC MATERIALS, SAID COMPOSITIONCONSISTING ESSENTIALLY OF A LOWER ALKYL-P-PHENYLENE DIAMINE AND A SULFURCOMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKYL MERCAPTANSCONTAINING FROM 4 TO 12 CARBON ATOMS, FURFURYL MERCAPTAN, THE LOWERALKYL ESTERS OF THIOGLYCOLIC ACID, 2-MERCAPTO IMIDAZOLIDINE AND LOWERALKYL 2-MERCAPTO IMIDAZOLIDINES, SAID SULFUR COMPOUND BEING PERSENT INAMOUNT BY WEIGHT BETWEEN ABOUT 25 PER CENT AND ABOUT 50 PER CENT OF SAIDSYNERGISTIC ANTIOXIDANT COMPOUND.